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| Book/Report | FZJ-2020-00270 |
2000
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/23906
Report No.: Juel-3738
Abstract: During combustion or gasification of coals in power planes, alkali metals are released, which can cause high temperature corrosion of plant components in contact with the hot gas. The high temperatures envisaged in effcient combined cycle planes increase the damaging potential of the alkali metals and require measures for reduction of the alkali concentration in the gas. One approach to solve the problems is to reduce the release of alkali metals directly during coal conversion. This requires an enhanced basic understanding of the alkali release from coals and coal ashes and is the primary aim of this work. Vaporization investigations by Knudsen effusion mass spectrometry, which enables the analysis of the gas phase over coal ashes in dependence on the temperature, form the centre of the basic experimental studies. At temperatures frone about 200 to 1800°C, altogether 24 alkali metal-containing and other volatile species could be detected with partial pressures between approximately 10$^{-5}$ and 100 Pa. The ash samples were produced by oxidizing the coals at 150°C in oxygen plasma as well as between 450 and 1200°C in the muffle furnace. The characterization of the samples was performed mainly by element analysis and X-ray diffraction as well as microscopic ash fusion tests. Some ashes were also examined for phase transitions and melting proeesses by high temperature X-ray diffraction and differential thermal analysis at temperatures up to 1200 and 1600°C respectively. The investigated materials are 7 lignites and 4 hard coals of different origin as well as 37 ashes produced in the laboratory from these coals. Furtherrnore, the corresponding ashes formed by combustion of one lignite and one hard coal in firing systems are considered. Additionally, the studies include hard coal slags enriched to different Na$_{2}$O and K$_{2}$O contents. As one of the most important results, different alkali release proeesses from lignite and hard coal ashes were detected. Typical for the first is that the alkali metals vaporize above 500°C as NaCl and KCl. From the latter, the alkali metals are released with comparable partial pressures (>0.1 Pa) only above 900°C and almost exclusively as Na and K. A further result is that the predominant portion of the alkali metals in lignites and in hard coals can remain bound in or absorbed by silicates in the mineral matter during oxidation in the laboratory furnace or the combustion process in a power plant and is not released. The degree of absorption depends mainly on the ratio of the amounts of silicate network-forrning (SiO$_{2}$, Al$_{2}$O$_{3}$) to network-changing components (chiefly CaO, MgO) in the ash and can be influenced by additives.
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